This invention relates to a hydraulic control apparatus in an automatic transmission for an automotive vehicle.
In general, an automatic transmission for an automotive vehicle is equipped with a planetary gear mechanism and a plurality of friction devices each comprising a clutch or brake. The engaging states of these friction devices are changed over in a variety of ways to selectively couple several rotary elements in the planetary gear mechanism with one another or brake the rotating motion of these elements, thereby automatically establishing the optimum speed with regard to the running state of the vehicle at such time. To this end, control for changing over the friction devices ordinarily is carried out by a hydraulic circuit. Specifically, control is performed by manually operating a manual lever in the passenger compartment to position a manual valve connected to the lever by a link and cable. The supply and discharge of oil pressure to and from each friction device generally is performed by a plurality of speed-changing shift valves, such as a 1-2 shift valve, 2-3 shift valve and 3-4 shift valve. Meanwhile, vehicle velocity and throttle opening signals conforming to the running state of the vehicle are fed into an electronic control circuit. The latter renders a decision upon making a comparison with a previously stored shift pattern and, based on the decision, actuates solenoid valves provided in the hydraulic circuit, thereby controlling the changeover of the aforementioned shift valves to achieve the gear best suited to the running state of the vehicle. The engaging pressure characteristic of each friction device is set by an accumulator.
Further, line pressure in the hydraulic circuit is controlled using a cable cam-type throttle valve or a linear solenoid.
The hydraulic circuit is so designed that if a solenoid should happen to become disconnected or severed, the circuit will function to assure, e.g., second gear in forward, the reverse gear or the neutral state, and mechanical locking will not occur even if the shift valves are changed over in all types of combinations.
Modern compact cars must be designed to have a compact automatic transmission possessing a number of speeds, i.e., three or four. Though the mechanical portion of the gear train and the like is designed to be compact, the size of the hydraulic control section controlling the speeds of the transmission generally is related to the number of speeds and is independent of the size of the gear train. As a result, it is difficult to make the hydraulic control section small in size.
In conventional automatic transmissions for automotive vehicles, the shift valves that are changed over in accordance with throttle opening and vehicle velocity, the throttle valve for controlling line pressure and the accumulators for setting the engaging pressure characteristics of the friction devices are incorporated within the transmission. When a large number of speeds are provided, therefore, the number of shift valves and accumulators required increases, the valve body becomes large in size and the hydraulic circuit becomes more complicated. In addition, weight, volume and cost increase. As a consequence, obstacles are encountered in reducing the size and cost of the automatic transmission.
An accumulator is for deciding the characteristic of engaging pressure by an orifice and spring force. To this end, the accumulator is adapted to perform precise shock control with respect to all shift conditions, such as throttle opening, vehicle velocity and oil temperature, and difficulties are encountered in tuning the accumulator to different types of automatic transmissions.
Furthermore, since three friction devices C0, C1, B1 are controlled at different times when making shifts from 2nd to 3rd and from 3rd to 4th, a large time lag is involved. Also, clutch pressure cannot be freely controlled. Consequently, when a speed change is made by changing over engagement between clutches or between clutches and brakes without using a one-way clutch, a large shock is produced at shifting.